1. Field of the Invention
The present invention relates to a fuel injection system for supplying a fuel to an internal combustion engine and, more particularly, to a fuel control device for a fuel injection system of the type that injects a fuel intermittently.
2. Description of the Prior Art
This type of fuel injection systems is operative to produce pulse signals according to the operating condition of an associated internal combustion engine and to deliver these signals to an electromagnetic fuel injector so that the internal combustion engine is supplied with the fuel intermittently in synchronization with the engine operation.
The intermittent fuel injection systems are sorted into two types, one is the Single Point Injection type and the other is the Multi-point Injection system.
The single point injection system has a single injector adapted for supplying the fuel to all or a half of the cylinders of the internal combustion engine, whereas the multi-point injection system is adapted to supply the fuel to the cylinders by means of injectors associated with respective engine cylinders.
In designing and manufacturing the intermittent fuel injection systems, it is necessary to overcome the following problems (1) and (2).
(1) If the valve-open period of the fuel injector is set to be 2 ms, for example, at an idling speed of 600 R.P.M., the valve-open period will be prolonged to 10 ms, which is longer than the valve-open period of the intake valves, as the engine speed is increased to 3,000 R.P.M. For this reason, it is necessary that the valve-open period of the injector be determined on the basis of the engine speed at the high-speed engine operation. This, however, causes an unsteady idling operation, because the fuel injection interval is inconveniently increased during the idling operation. More specifically, if the valve-open period of the fuel injector is selected to be 5 ms at 3,000 R.P.M., the valve-open period will be shortened to 1 ms as the engine speed is decreased to the idling speed of 600 R.P.M. This valve-open period is too short for the valve-opening period of the intake valves during the idling operation which is typically 50 ms. Under such a condition, there is a considerably long period in which air is supplied solely, after each fuel injection, so that the whole intake air is not mixed with the fuel homogeneously to make the idle operation unsteady.
(2) It is desirable that the fuel injected from the fuel injector is atomized into fuel particles of particle sizes as small as possible, because the smaller particle size ensures a better driveability and emission control, as well as reduced fuel consumption rate. The particle size of the fuel particles injected from conventional fuel injectors is around 300.mu. which cannot be considered sufficiently small. Therefore, part of the fuel taken into the engine attaches to the wall of the cylinder and is emitted from the latter before it is burnt. The fuel attaching to the cylinder wall, on the other hand, dilutes the lubricating oil and increases the fuel consumption rate uneconomically.
Under these circumstances, the specification of U.S. Pat. No. 3,656,464 proposes a system intended for overcoming the above described problems of the prior art.
This system, however, is intended merely to effect a finer atomization of the fuel, and cannot eliminate the aforementioned considerably long period of air supply after each fuel injection during idling operation of the engine. Thus, this system cannot overcome the above-explained problem (1) of the prior art.
It is also to be pointed out that this system can atomize the fuel only to the order of 50 to 70.mu., which is considered still insufficient, particularly in view of current social concern about the exhaustion of oil resources.